1. Field of the Invention
The present invention relates to a transmission cable structure and, in particular, to a transmission cable structure used in high frequency transmission system, which can restrain loop noise and ensure the symmetry of transmission cable.
2. Description of Related Art
Due to functions of the central processing unit of a computer having been upgraded, the net bandwidth increasing and storage medium data rising tremendously, both the input and output interfaces of the periphery are getting necessary to provide larger bandwidth so that the standard for wide band transmission cables are regulated largely like bamboo shoots after spring. However, problems, such as transmission delay, impedance matching, cross talk, and ground noise control and electromagnetic wave radiation interference, resulting from high frequency signals are getting serious in the transmission cables along with the increased bandwidths of the transmission cables.
As for the transmission interfaces in a computer, Serial ATA, the serial transmission equipment, is a transmission cable providing with the highest bandwidth at the present time. Because the Serial ATA has two conductors, i.e., a transmission pair, being used as a transmission channel set for transmitting differential NRZ signal, the conductors in the same transmission channel being very much different from each other in their lengths causes signals transmitted at the same time from the input end being not possible to reach the output end simultaneously in the process of signal transmission. That is, the signals emitted from the input end are possible to be received asynchronously, and becomes the so called problem xe2x80x9cpropagation skewxe2x80x9d.
Furthermore, in a transmission system, due to the transmitting end and the receiving end have different potential; so there exists a potential drop between the transmitting end and the receiving end. The transmission system will generate a current and form a noise source due to the potential drop; therefore, for controlling the noise being generated, the transmission lines are separated from (1) signal ground loop and (2) huge current ground loop (such as ground of the chassis or frame), by separating the signal ground loop and the huge current ground loop to ensure the level of the transmission signal without effecting by coupling voltage of the other noise sources, besides, can canalize the leakage current of the chassis and prevent accident being occurred.
The U.S. Pat. No. 6,444,902, published on Apr. 20, 2001 related to the currently used transmission cable structure, mainly has two transmission channel sets includes two independent conductors, a drain wire disposed at two opposite lateral sides of the transmission channel set, an inner covering providing an effect of shield and wrapping both outer sides of the conductors and the drain wire with a conductive layer and an insulation layer from the inner side to the outer side thereof and a jacket covering the inner covering to constitute a high frequency cable.
But, the preceding transmission cable has the following problem in practice:
(1) Transmission propagation problem: due to the signal conductors of the transmission channel set without any location structure, so during being fabricated, the transmission cable is coiled up; but, the two conductors in the transmission channel easily displace because of being bent during the process of coiling such that it is not easy to control the conductors in their lengths in case of being cut it results in an signal transmission propagation and asymmetry due to the two conductors being unequal in their lengths such that the bandwidth of the transmitting frequency is limited and cannot transmit higher frequency signal.
(2) Noise control problem: due to the transmission channel set only disposed a metal shield outside the two signal conductors, only the transmission signal has a ground loop, and doesn""t separate the ground loop from the huge current ground loop, thus, the huge current noise from chassis or frame will couple to the signal ground loop, so as to effect the electrical characteristic of the transmission signal and results in higher signal to noise ratio or transmission error.
Accordingly, an object of the present invention is to provide a transmission cable structure with a double shield effect and to secure the conductors in every propagation channel being kept juxtaposed and equal lengths even if the transmission cable is bent instead of the two conductors being misplaced and unequal as the prior art does so that the propagation delay at the output end can be reduced to enhance the bandwidth of the signal in the transmission cable and to improve the electrical characteristic of the transmission signal.
In order to reach the preceding object, the transmission cable structure comprises: a propagation channel set, which further comprises a pair of insulated conductors for carrying out communication of electrical appliances; a first covering, for enclosing an outer side of the propagation channel set so as to locate two insulation conductors and to form an electrical shield; at least one drain wire, being disposed outer side of the first covering and electrical connection to the first covering; a second covering, for enclosing the first covering and the drain wire, wherein, the second covering comprises at least one protection layer for forming an electrical shield; and a jacket, being an outermost layer for protecting the cable.